Certain existing emergency ballasts, e.g. for driving multiple lamps, provide for the electronic ballast to be disconnected from the mains in the case of a mains failure. One of the lamps driven by the electronic ballast is disconnected (e.g. via a relay) from the electronic ballast and coupled to a high-frequency supply source of an emergency ballast. Typically, the lamp is driven in a dimmed condition for reducing power consumption, and the emergency supply continues until the mains feed is re-established or the battery of the emergency ballast is exhausted.
Such an approach works quite well for non-dimmable ballasts but exhibits drawbacks during normal operation of dimmable ballasts and may be the source of complaints and requests for assistance in-the-field.
For example, such arrangements may result in an additional asymmetric capacitive load for a dimmable electronic ballast, which may lead to a non-uniform distribution of currents between the lamps in dimmed operation. In extreme cases, the lamp having coupled thereto the emergency ballasts can also be completely turned-off. Also, wiring errors occur quite often leading to increased field claims.
Increased awareness of energy efficiency in lighting applications is driving the use of dimmable ballasts and lighting control instead of non-dimmable solutions. Therefore an improved emergency ballast is highly favourable which is overcoming the limitations of state of the art designs, if operated with dimming ballasts.
Some prior art solutions try to overcome this problem by providing an emergency power supply circuit which feeds directly the existing electronics ballast in case of a mains failure.
For example, document DE 202 19 737 U1 discloses such an emergency power supply circuit, which however does not provide for dimmed operation.
Document DE 202 05 234 U1 discloses an emergency power supply arrangement relying on a “central” emergency battery, while also providing for dimming operation.